Diamond and diamond-like substances have many properties, such as wear resistance, thermal conductivity, acoustic transmission, and corrosion inertness, which make them desirable for a variety of industrial applications. To this end, diamond and diamond-like substances have been incorporated into tools for various purposes such as saw blades, drill bits, and electronic components such as surface acoustic wave filters. One method for incorporating diamond or diamond-like materials into a tool is known as chemical vapor deposition (CVD).
Various CVD techniques have been used in connection with depositing diamond or diamond-like materials onto a substrate. Typical CVD techniques use gas reactants to deposit the diamond or diamond-like material in a layer, or film. These gases generally include a small amount (i.e. less than about 5%) of a carbonaceous material, such as methane, diluted in hydrogen. A variety of specific CVD processes, including equipment and conditions, are well known to those skilled in the art.
In forming a layer of diamond, or diamond-like material on a substrate using CVD techniques, a plurality of diamond grains, or “seeds,” may be first placed upon the substrate surface. The placement of such seeds may be accomplished using CVD itself such as by applying a voltage bias, by polishing with micron-sized diamond, or by other methods known in the art. These seeds act as diamond nuclei and facilitate the growth of a diamond layer outwardly from the substrate as carbon vapor is deposited thereon. As a result, the growing side of the diamond layer becomes increasingly coarse in grain size, and must ultimately be ground and polished to a smooth finish such as by a mechanical means, in order to be suitable for many industrial applications. However, as diamond and diamond-like substances are among the hardest known materials, such mechanical grinding and polishing is difficult and tedious. Moreover, the cost of polishing often exceeds the cost for making the diamond film itself. In addition, mechanical polishing inevitably introduces micro-cracks or variations on the diamond surface. Such cracks and variations are detrimental to certain applications.
For example, if the diamond is used make a surface acoustic wave (SAW) filter, micro-cracks or variations in the diamond surface will introduce noise into the signal and therefore deteriorate the quality of the filtered wave. Diamond has a surface acoustic wave velocity of about 11 km/sec which is significantly higher than most materials commonly used in SAW filters such as quartz, lithium titanate, lithium niobate, langaside, zinc oxide, aluminum nitride and others. Typical methods for producing diamond-containing SAW filters involve polishing the diamond to create an even surface. However, these methods introduce surface defects which adversely effect performance. Typical surface roughness for these types of methods may approach about 3 nanometers (nm). As a result, as wavelengths decrease, the quality of the filtered wave decreases dramatically. For example, a wave produced using an interdigital transducer (IDT) spacing of about 1 micron traveling across a surface having a roughness of 3 nm results in a surface roughness which represents about 0.3% of the wavelength. As commercial pressures drive applications to use higher frequencies in order to accommodate greater amounts of information, the surface roughness will be a significant limiting factor.
As such, a process for making diamond containing devices which require little or no post synthesis work to achieve a finished product, and which contain a surface having an extreme smoothness, continues to be sought through on-going research and development efforts.